Coagulation abnormalities including recurrent thrombosis, fetal loss, thrombocytopenia and cerebral involvement ("Antiphospholipid syndrome", APS) in systemic lupus erythematous (SLE) are associated with the production of anticardiolipin antibodies (ACA) and lupus anticoagulant (LAC) activity. Together, these two groups of antibodies are referred to as antiphospholipid antibodies (APA) and are composed of a mixture of immunoglobulins. It is this heterogeneity that may be the source of the conflicting data on the immunological property and role of APA in the clinical manifestations of APS. Since some APS patients with elevated titers of APA do not experience thrombosis, we hypothesize that only certain APA with unique binding property may cause thrombosis. However, such APA may be necessary but not sufficient to induce thrombosis. In order to test this, we plan to do the following: 1) Generate by the combinatorial library approach a large panel of IgG APA Fab fragments from APS patients with elevated titers of serum APA and recurrent thrombosis; classify these Fab fragments into subsets according to their binding properties. 2) Convert the selected Fab fragments into intact IgG molecules; characterize and compare the binding properties of intact immunoglobulins with those of Fab fragments generated in aim 1. 3) Determine the functional properties of recombinant monoclonal IgG APA by in vitro blood clotting assay and in vivo murine model of thrombosis. 4) Sequence Ig V genes of representative monoclonal Fab fragments to delineate the possible structural basis for various binding properties and biological activities. The results of these studies will help delineate which APAs are likely to cause thrombosis and may lead to a better prediction of thrombogenic events in APS patients.